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Received this comment about the 5 pages of information provided about Modifying Holley Carbs:

Holley Carb for a Big Block Chevy

Purchasing a ZZ 502/502 from Chevy gets a "special" 850 Holley Carb, one would think it was dyno tuned to match this excellent combination of  engine components. Well if it was, perhaps at max throttle and power, it sure was far off at cruise and when accelerating in a relatively light Street Rod that really hooks up!!  I have had a number of ZZ502 owners indicate similar tuning problems.

Bottom Line

It was necessary to swap the Holley 850 Vacuum Secondary Carb that came with the Chevy 502/502 motor kit with a Holley 850 Double Pumper to achieve, on rapid acceleration,  the complete elimination of a Bog due to a lean fuel/air mixture or a Hesitation if the secondaries were set to open slow enough to prevent the Bog .  After having spent months trying to make the Vacuum Secondary Carb work, the use of an air/fuel meter clarified the decision to switch carbs .  For details about the meter and how is was invaluable, click on AIR/FUEL METER DETAILS.  

NEW INFORMATION Hot Rod Magazine, July 2005,  had an interesting article about a new product that validates some of the unique modifications needed for the "Double Pumper," check it out.

"Double Pumper" Modifications

Even the Double Pumper purchased (Holley # 4781C) needed quite a few modifications to operate on this Street/Performance ZZ 502 engine in this relatively light Pro Street 1934 Ford.  The car only weights 3000 pounds with 53% of the weight on the rear 16 inch wide Mickey Thomson's.  It gets a very good bite and super acceleration. It is necessary to consider the effects of rapid acceleration and quick RPM increase on the fuel delivery system .  The modifications include;

On the Primary:

1. Swapping the 30 cc front accelerator pump with a 50 cc Pump,

2. Use of a Yellow accelerator pump Cam,

3. Increasing Squirter Size, 

4. Installing High Flow Squirter Screws,

5. Switching to a lower opening value Power Valve,

6. Decreasing the Jet Size to achieve the proper cruising air/fuel ratio,

7. Increasing the Power Valve fuel passage diameter to compensate for the smaller Jets

On the Secondary:

1. Plugging the Power Valve fuel passage,

2. Increasing the Jet Size to compensate for the plugged Power Valve,

3. Installing Jet Extensions to move the Jets to the rear of the bowl to assure they are immersed in fuel at all times,

4. Evaluated various Squirter Sizes,

5. Use of a Brown accelerator pump Cam to provide enough fuel quickly when accelerating,

6. Installing High Flow Squirter Screws,

7. Replaced the fuel bowl vent with higher tube and a fuel bowl "Whistle" to eliminate the chance of fuel spilling into the carburetor throat when decelerating.

Details of all the changes made and test results from adjustments to the Vacuum Secondary Carburetor are provided on the site.  What the tuning details?

Modifying Vacuum Secondary 850 Carb

The ZZ 502/502 engine kit came with a Holley 850 CFM vacuum secondary carburetor.  One would think it was dyno-tuned by Chevy or Holley since it was part of they kit.  Well if they did, it didn't help at all with the transient performance when accelerating or even cruising in a light Street Rod!  In any instance, tuning on an engine dynamometer or even a chassis dyno is not sufficient for a car pulling better than a "G" acceleration off the line!  What do you think happens to the fuel in the rear bowl under those conditions?  We'll explain.

Although the vacuum secondary Carb was ultimately replaced with a Double Pumper  a great deal of time was spent and improved results achieved with the Carb supplied.  Some may find the modifications and improvements sufficient for their needs so we will describe them in detail.  See details of modifying original Vacuum Secondary Holley 850 Carb.

Modifying Holly 850 Double Pumper

It was ultimately necessary to replace the Vacuum Secondary Carb with a Holley 850 Double Pumper to achieve optimum results.  Even it needed significant modifications. See details of modifying the Double Pumper Holley 850 Carb. 

The information in this site summarizes the extensive experience in making a Chevy 502/502 Big Block perform for street/strip type use in a relatively light Street Rod.  It covers what none of the references detailed to make the system work to its optimum.  That we have achieved.

Note: This site is NOT meant to be a source for understanding the principles of how a Holley or for that mater how any carburetor works.  There are a number of books and some Internet sites that can provide that information (see  References, Tools and Background )

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